Automatic and semiautomatic transmissions, the latter of which are defined for purposes of the present invention as transmissions having a number of manually selectable gear ratios and a number of automatically selectable gear ratios, are known and widely used in the automotive and heavy duty truck industries. In vehicles including such transmissions, a control computer is typically provided for controlling the shifting between the automatically selectable gear ratios in accordance with various engine and vehicle operating conditions.
One specific embodiment of a known semiautomatic transmission used with heavy duty trucks is commonly referred to as a "TOP-2" transmission, wherein a number of the numerically high gear ratios (i.e. numerically lower gears such as 1st, 2nd, 3rd, etc.) are manually selectable, and shifting between the numerically lowest two gear ratios (i.e. numerically highest, or Top-2, gears) is managed by a control computer pursuant to a gear shifting control algorithm. Examples of such TOP-2 transmissions and control algorithms therefore are given in U.S. Pat. Nos. 5,393,276, 5,393,277, 5,401,223, 5,498,195, 5,591,102, 5,609,548 and 5,638,271 to White et al., each of which are assigned to the assignee of the present invention, the disclosures of which are incorporated herein by reference.
In controlling shifting between the automatically selectable gear ratios of such a semiautomatic transmission, the control computer is typically operable to vary the automatic downshift points (i.e. the engine RPM at which the control computer automatically shifts to the next lower gear) in accordance with certain engine operating conditions. FIG. 3 illustrates an example of one prior art downshift point modulation scenario 85, wherein a control computer is operable to vary the downshift points from an automatically selectable top gear of a semiautomatic transmission to the next lower automatically selectable gear. Under high throttle percentage conditions such as when encountering a steep grade, the control computer sets the downshift point at 1250 RPM to thereby maintain a high engine RPM for high throttle operation. Under a low throttle percentage conditions, the control computer moves the downshift point down to 1200 RPM. Finally, if the engine includes an engine brake system, the control computer moves the downshift point up to 1300 RPM whenever the engine brakes are enabled and the throttle percentage is below some low throttle percentage level, to thereby maintain a higher engine RPM for optimal engine brake operation.
Although the foregoing downshift modulation scenario provides for satisfactory operation under low throttle and active engine brake operating conditions, the downshift scenario for high throttle operation has certain drawbacks associated therewith. For example, when the vehicle approaches a small hill, it may be preferable from the driver's perspective to forego a downshift if the vehicle can traverse the small hill above some acceptable engine RPM level. Implementation of such a shift strategy requires moving the high throttle downshift point to a lower engine RPM. Such a downshift scenario results in "later" downshifts and in some cases results in no downshift at all if the vehicle can traverse the small hill at some reduced, but acceptable, engine RPM. Conversely, when the vehicle approaches a steep grade, it is preferable from the driver's perspective to downshift "earlier" in order to maintain higher average engine RPM and thereby minimize the total number of shifts needed to traverse the steep grade. Implementation of this shift strategy requires moving the high throttle downshift point to a higher engine RPM level. While both of these downshift scenarios are desirable, both represent mutually exclusive shifting goals using known downshifting criteria.
Designers of engine and transmission control systems have heretofore addressed the foregoing problem by providing a system that raises the downshift RPM point for assisted hill climbing if a driver manually activates the system, typically by modulating the accelerator pedal in a predetermined manner (e.g. by releasing the accelerator pedal and thereafter applying 100% throttle within some predefined time period). One problem with this approach, however, is that downshift point modulation requires driver intervention. Drivers may thus tend to use this feature when it is not needed, thereby resulting in degraded fuel economy.
What is therefore needed is a system for electronically controlling automatic transmission downshift points that is sensitive to variations in the grades encountered by the vehicle. Preferably, such a system should lower the downshift RPM point when encountering a small grade and should raise the downshift RPM point when encountering a steep grade, thereby maximizing fuel economy and minimizing the total number of shifts.